This invention relates generally to xerographic copying apparatus and, more particularly, to a contact fusing system and cleaning mechanism therefor for fixing electroscopic toner material to a support member.
In the process of xerography, a light image of an original to be copied is typically recorded in the form of a latent electrostatic image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic marking particles, commonly referred to as toner. The visual image can be either fixed directly upon the photsensitive member or transferred from the member to a sheet of plain paper with subsequent affixing of the image thereto.
In order to permanently affix or fuse electroscopic toner material onto a support member by heat, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material become tacky and coalesce. This action causes the toner to be absorbed to some extent into the fibers of the support member which, in many instances, contitutes plain paper. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be firmly bonded to the support member. In both the xerographic as well as the electrographic recording arts, the use of thermal energy and the simultaneous application pressure for fixing toner images onto a support member is old and well known.
One approach to this type fusing of electroscopic toner images onto a support has been to pass the support with the toner images thereon between a pair of pressure engaged roller members, at least one of which is interally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner images contacting the heated roll to thereby effect heating of the toner images within the nip. By controlling the heat transferred to the toner, virtually no offset of the toner particles from the copy sheet to the fuser roll is experienced under normal conditions. This is because heat applied to the surface of the roller is insufficient to raise the temperature of the surface of the roller above the "hot offset" temperature of the toner whereat the toner particles in the image areas of the toner would liquify and cause a splitting action in the molten toner to thereby result in "hot offset". Splitting occurs when the cohesive forces holding the viscous toner mass together are less than the adhesive forces tending to offset it to a contacting surface such as fuser roll.
However, toner particles will be offset to the fuser roll by an insufficient application of heat to the surface thereof (i.e. "cold" offsetting); by imperfections in the properties of the surface of the roll; or by the toner particles insufficiently adhering to the copy sheet by the electrostatic forces which normally hold them there. In such a case, toner particles may be transferred to the surface of the fuser roll with subsequent transfer to the backup roll during periods of time when no copy paper is in the nip.
Moreover, toner particle can be picked up by the fuser and/or backup roll during fusing of duplex copies or simply from the surroundings of the reproducing apparatus.
One arrangement for minimizing the problems attendant the foregoing, particularly that which is commonly referred to as "offsetting" has been to provide a fuser roll with an outer surface or covering of polytetrafluoroethylene, commonly known as Teflon (Trademark of E. I. Dupont), to which a release agent such as silicone oil is applied, the thickness of the Teflon being on the order of several mils and the thickness of the oil being less than 1 micron. Silicone based oils, which possess a relatively low surface energy, have been found to be materials that are suitable for use in the heated fuser roll environment where Teflon constitutes the outer surface of the fuser roll. In practice, a thin layer of silicone oil is applied to the surface of the heated roll to thereby form an interface between the roll surface and the toner images carried on the support material. Thus a low surface energy layer is presented to the toner as it passes through the fuser nip and thereby prevents toner from offsetting to the fuser roll surface. Also, stripping of the copy sheets from the fuser roll is facilitated. The foregoing notwithstanding, "non-visual offsetting" (i.e. offsetting of very fine particles of toner) does occur. In prior art constructions (fuser structures where the outer surface comprises Teflon or silicone rubber) such offsetting has been combated by the employment of various cleaning members, the wick material employed for applying the silicone based oil to the fuser roll serving this purpose.
Toner which is picked up by the backup roll, in whatever manner, is conventionally removed therefrom by means of a Nomex or similar type cleaning pad supported in rubbing contact with the backup roll. Nomex is a trademark of E. I. Dupont for a heat resistant nylon comprising the copolymer of meta-phenylenediamine and isophtalozl chloride. The backup roll comprises an unheated roll having a relatively thick layer of silicone rubber bonded to a core and having a sleeve of Teflon thereover. It is believed that the toner is readily removed from the Teflon outer surface of the backup roll by means of the Nomex pad due to the presence of copious amounts of silicone oil transferred to the backup from the heated fuser roll.
In the interest of improving at least the perceived copy quality of fixed toner images on plain paper, Teflon coated fuser rolls have given way to ones which are coated with a relatively thick layer of silicone rubber. Such rolls used in conjunction with a much harder backup roll have come to be referred to as Nip Forming Fuser Roll (NFFR) fusers.
Using the aforementioned Nomex cleaning pads in NFFR fusers for removing toner from the backup roll, it was found, required the application of higher loads in order to produce acceptable cleaning of the backup roll. While the cleaning was acceptable, it appeared that the higher loading employed adversely affected the wear rate and, therefore, the useful life of the pad. Moreover, due to the physical makeup (i.e. tightly packed fibers) of the pad the toner became imbedded in the surface of the pad thereby adding to the shortening of the useful life of the pad for the purpose of cleaning toner from the backup roll.